Socket Select problem

This is a discussion on Socket Select problem within the Networking/Device Communication forums, part of the General Programming Boards category; Hi I'm having a little trouble with the select and fork for the server side program that I'm building. The ...

  1. #1
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    8

    Socket Select problem

    Hi

    I'm having a little trouble with the select and fork for the server side program that I'm building. The problem is I got the socket and everything working and the basic problem can receive and display the data out. But whenever I put in the select with the fork, the program does not receive and display the data anymore. Can someone point me into the right direction of the problem that I have with the code?

    sd is the listening socket descriptor and sd2 is the accepted socket descriptor.

    Code:
    while (1) {
    
       		
    		result = select(sd, &fds, NULL, NULL, NULL);
    		printf("%d", result);
    		if (result > 0) 
    		{
    			printf("entered");
    	 		if (FD_ISSET(sd, &fds)) 
    			{
    
    				alen = sizeof(cad);
    				if ( (sd2=accept(sd, (struct sockaddr *)&cad, &alen)) < 0) {
    					fprintf(stderr, "accept failed\n");
    					exit(1);
    				}
    
    				//make child process
    				printf("forking now");
    				pid = fork();
    
    				if (pid < 0) {
    				        perror("fork");
    				        exit(1);
      				}
    
         				if (pid == 0)   //child process
    				{					
    
    					
    					/* The sd2 has data available to be read */
    					close(sd);
    					n = recv(sd2, buf, sizeof(buf), 0);
    					
    					printf("received size: %d", n);
    					printf("%d", buf);
    					close(sd2);
    				}
        				else		//parent process
    				{
    					printf("This is the parents");
    					close(sd2);
    				}
             				
       			}
    
    		}
    		else if (result < 0) {
    			/* An error ocurred, just print it to stdout */
    			perror("select");
    		}
    		else if (result == 0){
    
    			printf("select is resulting 0");
    		}		
    
    	}
    thank you for all the help

  2. #2
    Captain Crash brewbuck's Avatar
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    select() destroys the fds bitset and you never reinitialize it. You have to do it every time through the outermost loop.

    There might be other problems -- I haven't looked carefully.

  3. #3
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    Jun 2007
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    thx for the fast reply.

    I think I got the thing working with one little problem. Right now the code looks like the following with your modification suggestion.

    Code:
    	while (1) {
    
       		
    		result = select(sd+1, &fds, NULL, NULL, NULL);
    		FD_SET(sd, &fds);
    
    		if (result > 0) 
    		{
    
    	 		if (FD_ISSET(sd, &fds)) 
    			{
    
    				alen = sizeof(cad);
    				if ( (sd2=accept(sd, (struct sockaddr *)&cad, &alen)) < 0) {
    					fprintf(stderr, "accept failed\n");
    					exit(1);
    				}
    
    				//make child process
    				printf("forking now\n");
    				pid = fork();
    
    				if (pid < 0) {
    				        perror("fork");
    				        exit(1);
      				}
    
         				if (pid == 0)   //child process
    				{					
    
    					printf("this is the child\n");
    					/* The sdhas data available to be read */
    					close(sd);
    					n = recv(sd2, buf, sizeof(buf), 0);
    					
    					printf("received size: %d\n", n);
    					printf("%s\n", buf);
    					close(sd2);
    				}
        				else		//parent process
    				{
    					printf("This is the parents\n");
    					close(sd2);
    				}
             				
       			}
    			else
    			{
    				printf("WTF");
    				
    			}
    
    		}
    		else if (result < 0) {
    			/* An error ocurred, just print it to stdout */
    			perror("select");
    		}
    		else if (result == 0){
    
    			printf("select is resulting 0\n");
    		}		
    
    
    
    
    
    	}

    I got the server receive something from the client successfully, but I can open receive the msg one time and then it keep printing me bad file descriptor. I tried removed the while loop and it works perfectly fine( print one time). Is there something wrong I need to watch out for as well?

  4. #4
    Registered User
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    problem solved.

    it was a silly mistake...forgot to check the EINTR.

    thank you for all the help

  5. #5
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by saipkjai View Post
    problem solved.

    it was a silly mistake...forgot to check the EINTR.

    thank you for all the help
    Eh? Why were you receiving a signal?

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